In the processing of photosensitive material such as film or photographic printing paper, the exposed material is typically conveyed through a series of processing tanks each of which stocks a processing solution to perform a step in the processing. For example, the tanks may contain developer, bleach, fix and water in sequence.
As the photosensitive material passes through each of the tanks, the processing solution in the tank is absorbed into the photosensitive material and is thereby consumed. In addition, the processing solutions are oxidized by air and become exhausted through depletion of active ingredients and build-up of reaction products. Thus, replenishing solutions with the same compositions or more active compositions as or than those stocked in the processing tanks must be supplied. Otherwise, the operation of the processing system will vary with time leading to undesirable variation in the system output.